There are various known processes for the synthesis of F32. The hydrogenolysis of F12 (dichlorodifluoromethane) or of F22 (patents JP 60-01731 and EP 508 660) has the disadvantage of being generally not very selective and of giving worthless methane as by-product. It has recently been proposed to produce F32 by fluorination of bis(fluoromethyl) ether (patent EP 518 506).
It is also possible to produce F32 by fluorination of methylene chloride (F30) using anhydrous HF. Many patents describe this reaction, claiming the use of catalysts such as Cr.sub.2 O.sub.3, CrF.sub.3, AlF.sub.3, Cr/carbon, Ni/AlF.sub.3 etc.
The difficulty in this reaction lies in the stability of the catalyst, which tends either to coke rapidly or to crystallize. The problem becomes very tricky if it is intended to combine a high space time yield and a good selectivity while maintaining good stability of the catalyst.
To reduce this deactivation it has been proposed to employ specific catalysts such as a mechanical mixture of alumina and chromium oxide (patent GB 821 211). This patent gives an example for the fluorination of methylene chloride, but the F32 space time yields obtained on this catalyst are low (&lt;200 g/h/l) and the cumulative duration of the tests is shorter than 5 hours.
More generally, during fluorination reactions, it is very often envisaged to inject oxygen or air continuously to lengthen the lifetime of the catalysts. Thus, patent JP 51-82206 claims the use of 0.001 to 1% of oxygen to maintain the activity of a catalyst containing chiefly chromium oxide and optionally other metal oxides. It is indicated that the use of more than 1% of oxygen results in the appearance of secondary reactions and it is therefore recommended to employ preferably from 0.005 to 0.1% of oxygen. In this patent the fluorination reactions are carried out between 100 and 500.degree. C. and preferably between 250 and 350.degree. C. In addition, it is stated there that, starting at 200.degree. C., the catalyst activity is maintained by the introduction of oxygen. Although this patent mentions, among the reactions, the fluorination of CCl.sub.4, CHCl.sub.3,CH.sub.2 Cl.sub.2,CCl.sub.3 F, C.sub.2 Cl.sub.6, C.sub.2 Cl.sub.4 and C.sub.2 H.sub.3 Cl.sub.3, the examples refer only to the fluorination of perhalogenated saturated materials (CCl.sub.4 and C.sub.2 Cl.sub.3 F.sub.3). It is known, however, that the reactivity of perhalogenated molecules is very different from that of the hydrogenated materials.
The latter, such as F133a (1-chloro-2,2,2-trifluoroethane) are sensitive to elimination reactions (loss of HCl or of HF) and to chlorination reactions, which result in the formation of worthless by-products. As patent FR 2 433 500 shows, the introduction of oxygen at the reaction temperature (generally higher than that employed for fluorinating perhalogenated molecules) can then result in a drop in selectivity.
Chromium oxide, well known as a fluorination catalyst, is also a good catalyst for the oxidation of HCl (U.S. Pat. No. 4,803,065 and U.S. Pat. No. 4,822,589). The oxygen introduced during the fluorination reaction reacts with the HCl formed to produce chlorine by the Deacon reaction. This chlorine can then easily cause a chlorination of the hydrogenated materials present in the reaction mixture. In the case of fluorination of F133a in the presence of oxygen, products of the F120 series (C.sub.2 HCl.sub.n F.sub.5-n) are thus chiefly formed. Besides the formation of chlorine, this Deacon reaction also produces water which, because of corrosion problems, is particularly undesirable in a fluorination process.
To overcome this disadvantage it has been proposed to employ some chromium-based mixed catalysts which make it possible to restrict the Deacon reaction. Thus, patent EP 546 883 shows that, in the case of bulk catalyst, the addition of a metal such as nickel allows the oxidation of HCl to be partially inhibited. A similar phenomenon is observed on Ni-Cr/AlF.sub.3 mixed catalysts (patents EP 486 333 and WO 93/25507).
With a similar objective in view, patent EP 328 127 proposes to carry out the reaction of fluorination of F133a to F134a on a catalyst containing no chromium. The recommended solids contain at least one metal chosen from cobalt, manganese, nickel, palladium, silver, ruthenium and aluminium.
Recently, after having shown that in the case of the reaction of fluorination of methylene chloride in the presence of oxygen the chromium catalysts were not very selective (formation of F22 and of halogenated ethane derivatives), patent JP 5-339179 has also claimed the use of catalysts devoid of chromium, which are specific to the synthesis of F32. These catalysts, such as CoCl.sub.2 /AlF.sub.3 or NiCl.sub.2 /AlF.sub.3, are highly selective and their stability is increased by additives chosen from the rare earths (La, Ce) or alkaline-earth elements (Mg, Ca, Sr). The lifetimes obtained in the presence of oxygen are considerable (150 days), but the space time yields of F32 are very low (&lt;10 g/h/l) and are not compatible with an industrial production of F32.